Liquid crystalline polymers (LCPs) have become important items of commerce, being useful as molding resins for general purpose uses, and more specifically in the electrical and electronics industries due to their thermal stability, chemical resistance, and other desirable properties. For many applications, the molding resins should exhibit good stability when they are briefly heated, as well as when they are kept at high temperatures over extended time periods.
When aged at high temperatures LCP compositions exhibit one or more deleterious properties. Blistering is a phenomenon when gaseous inclusions (“bubbles”), especially larger inclusions, form within the polymer matrix. The observed result is undesirable blisters that are formed underneath the skin of the part. Thermooxidative degradation is a phenomenon exhibited by virtually all organic polymers, especially at higher temperatures and/or in the presence of oxygen. The rate of degradation depends on factors including temperature, the nature of the medium in contact with the part, the presence of damaging radiation, and the time that the part is exposed to this environment. While they often have excellent heat aging resistance compared to other thermoplastic materials, even LCPs sometimes suffer thermooxidative degradation, given severe enough conditions. Such degradation often leads to a gradual weight loss from the polymeric component, leaving behind inert components such as fillers. Other physical properties such as toughness, tensile strength and elongation at break often decrease in parallel with this weight loss, and these are therefore often measurable indicators for thermooxidative degradation. The rate of thermooxidative degradation is normally progressively higher at higher temperatures, typically following an exponential Arrhenius-type correlation with temperature.
Glass fillers such as fibers are used extensively to modify the physical properties. Such glass fillers are manufactured by a variety of processes, the most commonly used of which provides glass fiber in the form of bundles of many tens or hundreds of fibers each. The advantage of fiber bundles is that are more easily packaged and shipped, and are easier to subsequently feed into compounding equipment used to manufacture glass-reinforced plastics. To form these bundles, the glass fibers receive a very thin coating (“sizing”) after they are extruded, and this sizing holds the fibers together in these bundles, and protects the individual fibers from damage by rubbing against each other. Under high-shear conditions usually accompanied by heating, e.g. during compounding into thermoplastics, the bundles break apart and release the individual fibers into the plastic matrix. In addition to these functions, sizing on glass fillers intended for compounding into thermoplastics may also include a coupling agent which improves adhesion of the glass to the thermoplastic, for example with epoxides, silanes and the like. Glass filler such as glass fiber (particularly chopped glass fiber) and milled glass (fiber) used as fillers for thermoplastics almost invariably have sizing on them.
LCP compositions containing glass fibers without sizing are documented, see Japanese Patent Applications 08134334A, 07003137A, JP 05331356A, and U.S. Pat. No. 5,646,209. The compositions described in these references contain LCPs with melting points below 350° C., and/or also contain substantial amounts of other types of polymers which are not stable at higher temperatures. All of these documents are directed towards compositions which have improved physical properties such as tensile strength and elongation.